1. Field of the Invention
The present invention relates to a low-cost zoom lens system having a zoom ratio of approximately 3, and which is to be used in a small, light-weight digital camera. The present invention also relates to an electronic imaging apparatus which uses such a zoom lens system.
2. Description of Related Art
Along with rapidly increasing popularity of digital cameras in recent years, there has been a demand for lower-cost digital cameras, and there is also a strong demand for lower costs in regard to the photographing optical systems thereof. Furthermore, compact digital cameras which are miniaturized and made slimmer so as to have superior portability, tend to be more popular. On the other hand, every year the number of pixels of solid-state image sensors such as CCDs, etc., increases, so that a high-quality photographing optical system which is compatible with a fine pixel pitch is in demand.
A negative-lead type zoom lens system is often used in a zoom lens system having a zoom ratio of approximately 3 as a zoom lens system for a compact digital camera. Since a negative-lead type zoom lens system can widen the angle-of-view at the short focal length extremity and miniaturize the lens system (especially the diameter of the frontmost lens group), a negative-lead type zoom lens system is suitable for the so-called retractable zoom lens camera which accommodates the lens groups by reducing the distances therebetween when retracting the lens groups to an accommodating position. Furthermore, due to the need to position the exit pupil sufficiently far away from the imaging plane, a three-lens-group zoom lens system, i.e., a first lens group having a negative refractive power (hereinafter, a negative first lens group), a second lens group having a positive refractive power (hereinafter, a positive second lens group) and a third lens group having a positive refractive power (hereinafter, a positive third lens group) is often employed (e.g., see Japanese Unexamined Patent Publication Nos. 2005-84597 (Document 1) and 2007-193140 (Document 2)).
In Documents 1 and 2, relatively small negative-lead type zoom lens systems having a negative lens group, a positive lens group, and a positive lens group, in this order from the object, and a zoom ratio of approximately 3 have been proposed. However, these optical systems have long overall length and thick lens groups, which do not satisfy requirements for miniaturization. Furthermore, cost reduction is not sufficiently achieved. In a retractable zoom lens system, if an attempt is made to achieve further miniaturization in a fully-retracted state, in addition to reducing the overall length of the optical system and the lens diameter in a photographing position, the lens thicknesses of each lens group also needs to be reduced. However, such miniaturization is disadvantageous for the correcting of aberrations. Accordingly, it is a challenging problem to achieve the best possible balance between miniaturization and high specifications.